docs(signal): clarify container mode behavior

docs/channels/signal.md

@@ -229,6 +229,15 @@ The `apiMode` field controls which protocol OpenClaw uses:
 
 When `apiMode` is `"auto"`, OpenClaw caches the detected mode for 30 seconds to avoid repeated probes. Container receive is only selected for streaming after `/v1/receive/{account}` upgrades to WebSocket, which requires `MODE=json-rpc`.
 
+Container mode supports the same Signal channel operations as native mode where the container exposes matching APIs: sends, receives, attachments, typing indicators, read/viewed receipts, reactions, groups, and styled text. OpenClaw translates its native Signal RPC calls into the container's REST payloads, including `group.{base64(internal_id)}` group IDs and `text_mode: "styled"` for formatted text.
+
+Operational notes:
+
+- Use `autoStart: false` with container mode. OpenClaw should not spawn a native daemon when `apiMode: "container"` is selected.
+- Use `MODE=json-rpc` for receiving. `MODE=normal` can make `/v1/about` look healthy, but `/v1/receive/{account}` does not WebSocket-upgrade, so OpenClaw will not select container receive streaming in `auto` mode.
+- Set `apiMode: "container"` when you know the `httpUrl` points at bbernhard's REST API. Set `apiMode: "native"` when you know it points at native `signal-cli` JSON-RPC/SSE. Use `"auto"` when the deployment may vary.
+- Container attachment downloads honor the same media byte limits as native mode. Oversized responses are rejected before being fully buffered when the server sends `Content-Length`, and while streaming otherwise.
+
 ## Access control (DMs + groups)
 
 DMs: